The effects of reflected light have long been a common problem for professional and novice photographers alike. The light coming from the various objects in the scene to be photographed includes not only light having the color of the object but also light that may be reflected specularly from the object. While the specular reflection from an object, also called gloss or sheen, can add to the visual appeal of an object, for example the shiny finish of an automobile, the specular reflection of an object can also reduce the visual impact by reducing the apparent saturation, depth, or intensity of color of the object.
It is well known that light reflected specularly from the surface of an object can become at least partially polarized; that is the electric field vector of the reflected light tends to be oriented in particular direction. To the degree that the reflected light is polarized, it can be eliminated by placing a polarizing filter between the photographic media and the subject to be photographed. Polarizing filters are typically linear or circular.
When light is traveling in a particular direction, it has an electric field vector and a magnetic field vector. These field vectors are perpendicular to each other and to the direction of propagation. Light that is non-polarized has no preferred orientation for the electric field vector that is, its electric field vector, while perpendicular to the direction of propagation, can be at any azimuthal angle around the direction of propagation. When light has an electric field vector with a fixed orientation or azimuthal angle, the light is defined as being linearly polarized. Linear polarization commonly occurs in nature when light is reflected off of a flat surface. A linear polarizing filter allows only light having an electric field vector oriented a certain way, parallel to the polarization axis of the filter, to pass through. The amount of light incident on a polarizing filter that passes through the filter depends on the angle between the electric field vector of the light and the polarization axis of the polarization axis of the filter. For a polarizing filter to suppress the light polarized by a reflection off a surface, the polarizing filter must be oriented so that its polarization axis is perpendicular to the electric field vector of the reflected light. Thus, linear filters can block as much as 60% of the light from reaching the film.
The concept of circular polarization is similar to that to linear polarization. Circular polarization is a combination of two perpendicular linear waves that are 90 degrees out of phase with each other. Thus, a circular polarizing filter is nothing more than a linear polarizer, with the addition of a quarter-wave plate which converts the linear polarization of light passing through the polarizing filter to circular polarization. A circular polarizer is able to block all photons rotating in one direction while allowing those going in the other direction to pass.
It is well known that a polarizing filter is useful for reducing glare and by doing so producing more color contrast, but only if the polarizing filter has been rotated to the right orientation for the scene being photographed. For this reason, polarizing filters are screwed onto the front of the camera lens so that the photographer can rotate the filter to the desired angle. As the desired rotation angle of the filter can vary from scene to scene the photographer must observe the effect of the filter through the camera view finder while adjusting the filter rotation. While viewing a scene through the display or electronic viewfinder of a digital camera it can be hard to identify the preferred rotation of the filter as the image quality in the electronic display may not be sufficient to show the effect, and the display may be automatically “correcting” colors in the display.
While polarizing filters are useful for reducing unwanted reflections or glare from a photograph, there are other times in which it is desirable to photograph a scene without the use of a polarizing filter. One such case is when the reflections add to the visual appeal of the scene. Another case is in low light conditions. As polarizing filters can block up to 60% of the light they can increase the required exposure time to an unacceptable degree.